I dont want to rain on the TENS parade but I used it and it was not helpful. It distracted me but that was it..
That's the general idea. Its like when you bump your elbow and rub it - the rubbing distracts you from the pain and the sensation of the running gets to your brain before the pain receptors, lessening it.

TENs work by replicating that rubbing sensation, which travels to your brain faster than the pain can, so you need to time its use to your contractions too, a bit like if you have gas and air a lot of it is in the timing.